1. Field of the Invention
The present invention relates to a pneumatic fastener driving tool for driving fasteners such as nails, staples or the like to workpieces.
2. Description of the Prior Art
Pneumatic fastener driving tools are known, which include, as shown in FIG. 14 of the accompanying drawings, a driving rod 6 connected to a piston 3 and slidably received in a cylindrical barrel 2 for thrusting a fastener 1 which is retained in the barrel 2 in front of the driving rod 6. In the fastener driving operation, a tip end 13 of the fastener 1 is forced against a workpiece 12, thereby causing the driving rod 6 to move rearward (upward in this figure). This rearward movement of the driving rod 6 causes an O-ring 16 on the piston 3 to move upwardly past radial holes 18 formed in the peripheral wall of a cylinder 17, whereupon compressed air rushes through the radial holes 18 into a lower piston chamber 19 to lift the piston 3, thereby starting forward striking movement of the driving rod 6. Thus, the radial holes 18 constitute a start valve which is operative to commence or start the fastener driving operation. After the fastener driving operation, the piston 3 is normally at rest on a damper 4. However, when the driving force of the fastener driving tool is in balance with the resistance of the workpiece 12, or when the piston 3 bounds back from the damper 4 due to excessively large forces or pressure applied thereto, the piston 3 is separated from the damper 4 to such an extent that the start valve 18 is opened at least partly. This means that the pressure in the lower piston chamber 19 increases gradually and thereafter the driving rod 6 is suddenly thrust down again even after a cycle of fastener driving operation is completed. If such accidental fastener driving operation occurs during the manual loading of the next fastener 1, the operator might be seriously injured.
Another drawback of the conventional fastener driving tool is that the fastener driving depth can not be adjusted. When a fastener is to be driven into a workpiece until the head of the fastener lies flush with a surface of the workpiece, or when the fastener is to be driven into the workpiece to such an extent that the head of the fastener projects from the surface of the workpiece by a fixed distance for the purpose of hooking a wire or a fixture, the fastener driving operation using the conventional fastener driving tool must be followed by an additional manual hammering operation. The conventional fastener driving operation is laborious as a whole.
The conventional fastener driving tool has another problem arising when used in combination with hook-shaped fasteners such as shown in FIG. 14. In this instance, the hook-shaped fastener 1 is loaded in the barrel 2 with its leg la held on a side wall 2a of the barrel 2 and with a bent end 14a of the head 14 projecting outwardly from an axial groove 2b in the barrel 2. Since the leg la of the fastener 1 is out of alignment with the driving rod 6, the fastener 1 is likely to be deformed or bent as shown in FIG. 15 or tends to tilt as shown in FIG. 16. Such inadequately driven fasteners must be removed and replaced with a new fastener with the result that the fastener driving efficiency is considerably reduced.